Middle East respiratory syndrome coronavirus (MERS-CoV) is a newly emerging human health threat with a ~35% case fatality rate. MERS-CoV uses dipeptidyl peptidase 4 (DPP4), a cell surface protein, to enter and infect cells. During our Phase I study, using a transient plant expression system, we produced fusions of human DPP4 and the human immunoglobulin Fc sequences of IgG1, IgA1 and IgA2, to produce receptor decoys to block cellular infection with MERS-CoV. We demonstrated that DPP4-Fc binds to the S1 domain of MERS-CoV S protein, and that DPP4-Fc is a more potent inhibitor of MERS-CoV cellular infection than soluble DPP4. We showed that a DPP4-Fc fusion based on IgA1 might possibly be more effective than one based on IgG1. In addition, we demonstrated that binding and virus neutralization could be improved by more than 10-fold by modifying a single amino acid where human DPP4 and MERS-CoV spike protein contact. DPP4-Fc is also expected to have superior pharmacokinetics, as Fc will confer a long circulating half-life and the ability to be delivered to airway mucosal surfaces, the site of MERS-CoV infection. In a phase II study, we will produce new DPP4-Fc constructs to improve DPP4's affinity for MERS-CoV spike protein, and eliminate DPP4's peptidase activity by mutating the active site. We will optimize new constructs to improve expression in plants and to produce DPP4-Fc with human-like N-linked glycans. New DPP4-Fc fusions will be ranked for MERS-CoV spike protein-binding by ELISA and tested for the ability to neutralize MERS-CoV infection in susceptible cells. The best performing DPP4-Fc variants will be tested for protective efficacy in a mouse MERS model, evaluating both intraperitoneal and intranasal routes of administration. We will scale up production and purification of our lead molecule to purify 7 grams of DPP4-Fc, which will be used for a pilot 2-week repeat-dose safety/toxicology study in rats. We will evaluate safety and quantify DPP4-Fc in serum to obtain pharmacokinetic parameters. We will also screen for anti-DPP4-Fc antibodies in these studies.